Morphology: Formation of perfect stem

In English we use a compound form: The verb to have + past participle of the verb. Some participles are regular, i.e. they just add -ed to the verb itself (worked, walked, brushed etc. While others are irregular and have to be learnt separately: eaten, brought, swept, cut, written ....

 In Greek we use a simple form: special stem + perfect endings.
The endings are the same for all verbs, but the stem is not always predictable. Which means that we'll have to learn the present perfect form of each verb as we go along
 

Regular Formation

The verb-stem must end in a vowel. λύ-ω, φιλέ-ω, τιμά-ω, πληρό-ω, παιδεύ-ω etc.
In which case the perfect middle/passive endings are attached directly to the stem of the verb, whereas for the perfect active voice a κ is inserted between the stem and the endings.